Manufacture of ultra-clean surfaces by selective

ABSTRACT

A method and apparatus for removing surface impurities from a surface of a material, particularly silicon wafers, includes identifying the location of at least one impurity particle on a surface of the material and applying a liquid to the surface in the vicinity of the at least one impurity particle. The liquid is explosively evaporated to remove the surface particle as part of the evaporation of the liquid. The apparatus can include a source of humidified gas for the liquid and a laser apparatus to provide the heat for liquid evaporation.

This application claims priority under 35 USC 119(e) based onprovisional patent application no. 60/611,288 filed on Sep. 21, 2004.

FIELD OF THE INVENTION

The present invention is directed to an apparatus and method for theselective cleaning of impurity-containing substrates such as siliconsubstrates, and in particular, to a method which combines inspection andselective cleaning to improve subsequent manufacturing steps such asepitaxy or bonding, or SIMOX (=Silicon Implant by Oxygen ).

BACKGROUND ART

In the field of making silicon wafers for electronic component use, ithas been recognized and experimentally verified that surface particlescan be the source of defects in a subsequent manufacturing step. Aspecific example is shown in FIG. 1 wherein the normalized probabilityof a surface particle creating a defect in an epitaxial layer isdisplayed.

The pie chart of FIG. 1 was generated by measuring the X-Y coordinatesand size of a light scattering defect ( “LSD”) on a silicon wafer withan automated surface inspection equipment before and after an epitaxialdeposition step was performed on the wafer. The key in the pie chartlists the LSD sizes found on the substrate in microns. The probabilityof a substrate LSD for creating an epitaxial defect with a size greaterthan 1 micron by an optical surface inspection system was calculated bydetermining the percentage of the post epi defect X-Y coordinates thatcorrelate to a LSD location before the epitaxial deposition process.This analysis clearly shows that this probability scales with the LSDsize and if all these LSD's could be removed before the epitaxialdeposition, then the creation of an epitaxial defect should be greatlyreduced. In other words, manufacturing wafer surfaces with zeroparticles is a desirable condition for reducing defects. This result isnot only limited to the growth of defect-free epitaxial layers but alsocan be applied to other manufacturing steps, like the manufacture ofbonded wafers where any surface particles present lead to incompletebonding and to the creation of defects that are commonly known as voids.

The most common approach in the industry to alleviate these problems isto subject all wafers to a cleaning step before the next manufacturingstep. The most commonly used cleaning processes are those that removeparticles through chemical batch cleans that are highly tailored tospecific applications and typically assisted by directing bursts ofmegasonic pulses to the wafer surface. Typically, a batch of wafers issubmerged into a chemical bath where the wafers reside for severalminutes. These steps are repeated up to a few times and then thecleaning process is terminated by a rinse in an ultra-pure deionized(DI) water.

Another way to view the prior art technique is to gradually reduce thenumber of particles/impurities on the wafer. A first cleaning isperformed to reduce the particle count, for example to 10-30 particlesper wafer. This cleaning is typically a batch cleaning wherein a numberof wafers are aggressively cleaned to removes stains and impurityparticles from the surface. However, this cleaning step is usuallyinsufficient to prepare the surface for subsequent processing, and asecond cleaning step is generally needed. After the initial cleaning, asurface inspection is performed, and the rejected wafers are subjectedto a second cleaning wherein the particle count is further reduced, forexample to 5 particles per wafer. The second cleaning is typically asingle wafer treatment such as a brush or chemical spray treatment,although a batch cleaning could also be employed. Another surfaceinspection is made, and those wafers that do not pass are recleanedusing a similar technique or the same technique as used in the initialsecond cleaning. Wafers that do not pass another inspection afterrecleaning are discarded. Wafers that pass the third inspection aremoved to the next manufacturing operation.

One of the major drawbacks to these methods is the re-attachment ofparticles when the wafers transition through the liquid-air interface inthe clean bath. That limitation sets a general lower limit for theparticle level that can be achieved and presents a barrier for reachingzero particles.

As noted above, the prior art employs single wafer cleaning tools asopposed to the multiple wafer batch techniques mentioned above. Thesingle wafer techniques address the issue of particle re-deposition byapplying chemicals to a spinning wafer. In contrast to the batch methodswherein the wafers are moved, the treated wafers are stationary anddifferent chemistries are applied through a spray nozzle or a rotatingbrush. Although an improvement is seen in the number of LSD's presentwhen wafers are cleaned using single wafer techniques, random additionof particles still occurs.

Thus, there is a need for other methods of removing surface particles onsilicon wafers or other substrates so as to improve subsequentmanufacturing steps. It is the intention of the present invention toeliminate the occurrence of a low level and random deposition of thesesurface particles

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide an improvedmethod of reducing defect occurrences during the processing of siliconwafers.

Another object of the present invention is a method of removing surfaceimpurities from a surface, particularly a surface of a silicon wafer.

Yet another object of the present invention is an apparatus for removingimpurities from a surface of a material, particularly, a silicon wafer.

One other object of the present invention is a method and apparatus thatremoves surface impurities from a surface of an object using explosiveevaporation of a liquid.

Other objects and advantages will be come apparent as a description ofthe invention proceeds.

In satisfaction of the foregoing objects and advantages, the presentinvention, in one mode, comprises improvements in the methods andapparatus employed for cleaning the surface of material, especiallysilicon wafers. The invention is not limited to just silicon wafers, anymaterial having impurities that need removal for the material's use arecandidates for the invention. In one mode, the invention entails amethod of removing surface impurities from a surface. The method firstprovides a material having at least one impurity particle on a surfacethereof. A liquid is applied to the surface of the material in thevicinity of the at least one impurity particle. The liquid isexplosively evaporated such that the impurity surface particle isremoved as part of the evaporation of the water.

In a preferred mode, the explosive evaporation is accomplished by theuse of laser heat, wherein a laser beam is directed at theliquid/particle vicinity. The liquid can be derived from any source, buta preferred source is water placed in the vicinity of the particlethrough condensation as a result of a supply of a humidified gas.

While any means can be employed to remove the liquid-particlecombination, a preferred way is to employ suction at or near the site ofthe particle. The evaporation process can be monitored to ascertainwhether the particle has been removed. While a water-containing gas ispreferred as the source of the liquid, the liquid can be any water-basedcomposition, or alcohol, or an alcohol containing composition.

The method of cleaning is especially useful in the processing of siliconwafers, wherein the wafers are inspected for defects. In this aspect ofthe invention, a plurality of wafers are inspected for the presence ofsurface particles on a given wafer surface. From this set of wafers, anumber of wafers are selected that do not meet a pre-set inspectionparameter. For these wafers, a location of each of said surfaceparticles is identified and the selected wafers are subjected to thelaser cleaning step. After cleaning, the wafers can be subjected to afurther manufacturing step, with or without non-selected wafers.

The invention also includes an apparatus for removing impurities frommaterials, particularly but not limited to silicon wafers. The apparatusincludes a means for identifying the location of at least one impurityparticle on a surface of the material, and means for applying a liquidto the surface in the vicinity of the at least one impurity particle.Also provided as part of the apparatus are means for explosivelyevaporating the liquid, the impurity surface particle being removed aspart of the evaporation of the liquid.

The liquid applying means is preferably a gas applying means wherein thegas and particle are at a temperature such that the liquid in the gascondenses from the gas in the vicinity of the impurity particle to beremoved.

While any source of heat capable of evaporating the liquid can beemployed as part of the means for explosive evaporation of the liquid,it is preferred to employ a laser apparatus. The particle and liquid,once removed from the surface of the material, can be collected in anyfashion, with a preferred mode including applying a suction to thevicinity of the impurity particle. A monitoring device such as amicroscope or the like can be employed to monitor the particle and itssurrounding area to determine if the particle has been removed, or iffurther treatment is necessary.

Any source of liquid capable of being explosively evaporated can beemployed in the inventive method and apparatus for cleaning. A preferredliquid is water or a water-containing composition, e.g., a humidifiedgas, or alcohol or an alcohol containing composition.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIG. 1 is a pie chart showing the normalized probability of apre-epitaxial particle turning into a defect; and

FIG. 2 is a schematic of a unitary cleaning platform for wafertreatment;

FIG. 3 is a schematic of an apparatus according to the invention; and

FIG. 4 is a bar graph shows the effect of the laser cleaning of theinvention on the incidence of light scattering defects (LSDs) on a waferfor different size defects.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is concerned broadly with the treatment ofimpurity containing materials, and particularly silicon wafers inanticipation of manufacturing steps wherein other material is combinedwith the wafer. One example of this manufacturing is the creation of anepitaxial layer on the silicon wafer. Another example would be to createbonded or SIMOX wafers.

In one aspect, the present invention utilizes a two step approach forachieving a level of zero surface particles on silicon wafers. The firststep is to eliminate the re-cleaning of wafers that is described above.Instead of subjecting wafers to multiple cleaning, the invention treatsthe wafers that have been initially cleaned with the inventive two stepapproach of inspection and localized cleaning. That is, wafers that meeta pre-set criteria are transferred into a wafer carrier where they arestored for further processing. The pre-set criteria can be any number ofcharacteristics associated with the particles found on the wafersurface, e.g., particle size, number of particles per given wafer,number of particles in a given area, and the like. Any known means canbe used for identifying the particle and checking for the pre-setcriteria.

After inspection of the wafers, wafers that fail the pre-set inspectioncriteria are collected in a different carrier as a separate group andare sent for cleaning using the inventive localized cleaning technique.

Once the wafers are recleaned, they can be combined with the wafers thatpassed inspection for subsequent processing or can be used in subsequentprocessing on their own.

To demonstrate the advantages associated with the invention, anexperiment was conducted using wafers having zero surface particles.This group of wafers was run across a commercially available singlewafer brush cleaner followed by a re-inspection of the wafer surface tocheck for surface particles. It was discovered that only 85% of thewafers were particle-free after the brush clean, thus showing thatre-cleaning wafers does not necessarily result in improved performance.

According to one mode of the invention, the re-cleaning of the rejectedwafers is done using a localized cleaning technique employing laserheating. This cleaning platform incorporates two innovative approaches.First the surface inspection is integrated with a laser cleaning moduleinto one cleaning platform. The second innovation is the concept of alocalized cleaning using explosive evaporation of liquid, e.g. water,via laser heat to remove one or more particles identified and targetedfor treatment. It should be understood though that other sources of heatas would be known in the art could be employed that would cause theexplosive evaporation of the liquid, e.g., water, for surface particleremoval. A preferred method of explosively heating the liquid film isachieved within a spectrum of laser wavelengths (e.g. 2-5 microns),although other wavelengths can also function adequately, e.g., 10.2microns for a CO₂ laser. The wavelength should be selected so that isstrongly absorbed in the liquid film but not in the underlying silicon,thus preventing damage thereto. It is also preferred to use a nanometerscale pulse width (picosecond would damage the wafers and microsecondwon't have enough impact) and an energy flux of <1J/cm².

FIG. 2 shows a schematic of a unitary cleaning platform for wafertreatment designated by the reference numeral 10. The platform 10includes an inspection station 1, e.g., an Excite inspection station,and a cleaning station 3. In practice, a wafer enters the inspectionstation at 5 so that its surface can be inspected for the presence, sizeand/or distribution of one or more particles or otherimperfections/impurities that require removal. Once the particle orparticles are identified and their location is recorded, the particleparameters are compared to pre-set criteria such as particle size and/ordistribution, number of particles or the like. Virtually any criteriacan be used depending on the subsequent processing steps that willoccur. The identification of the surface particles to be treated can bedone using any known technique in the art, and a further description ofthis aspect of the invention is not necessary for understanding of theinvention.

Still referring to FIG. 2, if the wafer passes the inspection criteria,it follows path 6 for further processing. If the wafer does not pass theinspection criteria, it is moved to the cleaning station 3 via path 7.The cleaning station 3 has an inlet 9 to receive a cassette 11containing the wafers requiring treatment. The station 3 has aprealigner shown as box 13 that positions a wafer on a stage 15 forfurther precise alignment of the particle to be treated with thelocalized cleaning apparatus designated by the reference numeral 17.Although not shown, the stage can include a pre-alignment device forpositioning the wafer for cleaning. A handler robot or the likedesignated by the number 18 can also be provided for wafer manipulationin the cleaning station 3. The cleaning station 3 includes a filter fanunit shown as box 19 for controlling the atmosphere during the entry andexit to the cleaning operation. Once the wafer is cleaned, it exits thecleaning station via cassette 20. The cleaned wafer is then transferredalong path 22 for further processing via the standard mechanicalinterface (SMIF) loader 24.

In one mode, the concept of localized cleaning is accomplished asfollows:

-   -   An X-Y coordinate for each LSD greater than a pre-selected        threshold size is located and recorded for each inspected wafer        at the inspection station 1.

A stage 15 movable in the X-Y directions is provided, and this stagemoves the wafer such that the identified particle is positioned beneatha laser cleaning module of the cleaning apparatus 17.

-   -   A humidified gas is supplied in the vicinity of the LSD, e.g.,        in an impact area of about 1 mm², under appropriate temperature        control so that a water film is condensed around each LSD        individually.    -   The water film is directly heated with an infrared laser pulse        such that the particle is removed by explosive evaporation of        the water. The laser can be a conventional IR pulsed type that        would provide sufficient heat to explosively evaporate the water        around the particle. Any other type of a laser or other device        or means capable of providing the localized heating of the water        for the explosive evaporation can be employed.

Removal of the detached particle can be assisted by applying localsuction to the detached particle. Other removal techniques or means suchas applying pressure or mechanical shock to the area to direct the watervapor away from the wafer surface for collection can also be employed,or a general suction in the treating chamber to move the evaporatedwater away from the surface of the wafer can be utilized. Removal of theparticle does not induce any crystalline damage to the wafer beingtreated.

FIG. 3 shows an example of the cleaning apparatus 17. A wafer 21 isshown having a particle 23 on its surface. Although not shown, apparatusbeneath the wafer allows for its rotation and/or translation so as toposition a given particle for removal. Rotation motion is shown by thearrow “A” with translation represented by the arrow “B”. A processmonitoring microscope 25 or other observation means can be provided toallow monitoring of the operation and determination of whether theparticle was removed or not. If this observation determines thatcleaning has not occurred, the process can be repeated. This monitoringstep allows for iterative cleaning and enabling of judgment of thecleaning performance. Also, the monitoring does not require a separatewafer inspection step. After monitoring, the wafer can be classified ascleaned and sent for further processing, or classified as needingadditional cleaning, e.g., subjecting the particle to one or moreadditional explosive evaporation sequences, or another prior artcleaning, or classified as being rejected.

A pulsed infrared laser apparatus 27 is provided that includes anattentuator wheel 29 and focusing lenses and mirror 31. The laserapparatus supplies heat in the form of the laser beam 33 that isdirected at the water at or near the particle to cause the explosiveevaporation. Also present are means for applying a liquid in thevicinity of the particle to be removed. Water is a preferred liquid, andit is supplied by humidifying a dry nitrogen gas 35, and feeding the drygas through a humidifier 37. The humidified nitrogen gas 39 is directedalong path 41 to the vicinity of the particle to be removed. The gas andwafer temperatures are controlled or set so that the water in the gascondenses in the vicinity of the particle to be removed. A photodiode 43is provided to check for reflection of light from the applied water toensure existence of water at the particle vicinity for the evaporationstep. While water is supplied using a humidified nitrogen gas, othermethods/means could be used to apply the liquid to the vicinity of theparticle to be treated, e.g., merely applying water directly rather thanthrough condensation. Other gases besides nitrogen could be used thatwould be inert to the cleaning environment.

A dark field illumination laser 45 can be provided as well to illuminatethe area where the particle resides. Also provided is a localizedsuction device 47, a tip 49 thereof positioned in the vicinity of theparticle 23 so that the evaporated water with the removed particle canbe collected to prevent re-attachment of the particle to the wafersurface.

The advantages of the invention can be seen from the following twocomparisons. A first comparison involves the percent recovery of wafersthat have zero particles greater than 0.8 micron and less than 3particles greater than 0.3 microns. The current percent recovery ofprior art techniques wherein the wafer are subjected to a firstcleaning, inspection, a second cleaning, a second inspection, and are-cleaning, is about 72%. In contrast, where wafers are subject to thefirst cleaning, inspection and then cleaning according to the invention,the percent recovery is 85%, a significant increase over the prior artrecovery. In essence, the invention eliminates the recleaning step ofthe prior art technique.

A second comparison involves the distribution of the light scatteringdefects or LSD on a wafer. A comparison was made between a control groupof wafers that were subjected to two cleanings and two inspections,i.e., a first aggressive cleaning as described, inspection, a secondaggressive cleaning, and then the second inspection.

A second group of wafers was agressively cleaned, subjected to surfaceinspection, and rejects from this inspection were subjected to lasercleaning. The cleaned wafers were then subjected to a second inspectionfor comparison with the control group. In this comparison, the prior artaggressive cleaning methodology was compared to the laser cleaningtechnique of the invention.

FIG. 4 shows the results of the comparison between the invention andprior art in graphical form. The graph compares the normalized LSD countper wafer for the two groups in terms of LSD particle size. The lightlyshaded bars represent the prior art control group and the more heavilyshaded bars represent the group subjected to laser cleaning. As isevident from this graph, the normalized count for the inventiveprocessing is significantly lower than that of the prior art. Forexample, for LSD greater than 0.3 microns, the normalized count is lessthan 0.2 LSD per wafer for the invention as compared to 1.0 using theprior art cleaning. Improvements are also realized when consideringlarger defects. For example, for LSD greater than 1.0 microns, the priorart technique results in 0.4 LSD per wafer, whereas practicing theinvention results in an LSD count of almost zero. For LSD greater than10 microns, the prior art cleaning still leaves 0.2 LSD per wafer,whereas the inventive technique effectively removes all defects.

The control group and the laser-cleaned group were taken to epitaxialdeposition and the defects and epitaxy were measured for comparisonpurposes. The comparison of the defects is shown in the following Table.The Table categorizes the material by three levels of defects, andmeasures the yield for the control group of wafers, and the groupprocessed according to the ivnention. The yield in percent is shownusing the control group as the basis, the yield being normalized to thecontrol group, and the yields are characterized in terms of the numberof defects. TABLE Process Zero Defect Max = 1 Defect Max = 2 DefectControl 1.0 1.0 1.04 Invention 1.27 1.10 1.04

The Table demonstrates that a significant increase in yield is obtainedfor the laser cleaned wafers as opposed to the control wafers, with theyield increasing with the number of defects decreasing.

While water is exemplified as the liquid that would be explosivelyevaporated, other liquids could be used such as alcohol, or anaqueous-based liquid employing water and other substances, or analcohol-containing liquid, e.g., alcohol and water. Virtually any liquidthat would be capable of being heated in the vicinity of the particle orimpurity to be removed such that the liquid would explosively evaporateto remove the particle could be employed in the invention. It shouldalso be understood that the liquid could also contain additives such assurfactants and the like to enhance the particle removal.

As such, an invention has been disclosed in terms of preferredembodiments thereof which fulfills each and every one of the objects ofthe present invention as set forth above and provides a new and improvedmethod and apparatus for improving the surface quality of objects, andparticularly silicon wafers.

Of course, various changes, modifications and alterations from theteachings of the present invention may be contemplated by those skilledin the art without departing from the intended spirit and scope thereof.It is intended that the present invention only be limited by the termsof the appended claims.

1. A method of reducing defect occurrence in processing of siliconwafers comprising: inspecting a plurality of wafers for the presence ofsurface particles on a given wafer surface; selecting from saidplurality of wafers one or more wafers that have one or more surfaceparticles that do not meet a pre-set inspection parameter andidentifying a location of each of said surface particles; and subjectingthe selected wafers to a cleaning step, wherein the cleaning stepincludes providing a liquid in a vicinity of each detected surfaceparticle and explosively evaporating the liquid, each detected surfaceparticle being removed as part of the evaporation of the liquid, theselected wafers then subjected to a further manufacturing step, with orwithout non-selected wafers.
 2. The method of claim 1, wherein laserheat is used for the explosive evaporation.
 3. The method of claim 1,wherein a suction is applied in the vicinity of the detected surfaceparticle during the evaporating step to help remove the detected surfaceparticle.
 4. The method of claim 1, wherein the explosive evaporation ismonitored to ensure that the detected particle is removed.
 5. The methodof claim 1, wherein the liquid is provided by condensation from a gas.6. The method of claim 1, wherein the liquid is water or a water-basedcomposition, or alcohol, or an alcohol containing composition.
 7. Amethod of removing surface impurities from a surface comprising thesteps of: a) providing a material having at least one impurity particleon a surface thereof, b) applying a liquid to the surface of thematerial in the vicinity of the at least one impurity particle; c)explosively evaporating the liquid, the impurity surface particle beingremoved as part of the evaporation of the water.
 8. The method of claim7, wherein laser heat is used for the explosive evaporation.
 9. Themethod of claim 7, wherein the liquid is applied by condensation from agas.
 10. The method of claim 7, wherein suction is applied during theexplosive evaporation step to assist in impurity particle removal. 11.The method of claim 7, further comprising monitoring the explosiveevaporation step for particle removal.
 12. The method of claim 7,wherein the liquid is water or a water-based composition, or alcohol, oran alcohol containing composition.
 13. An apparatus for removing surfaceimpurities from a surface of a material comprising: a) means foridentifying the location of at least one impurity particle on a surfaceof the material, b) means for applying a liquid to the surface in thevicinity of the at least one impurity particle; c) means for explosivelyevaporating the liquid, the impurity surface particle being removed aspart of the evaporation of the liquid.
 14. The apparatus of claim 13,wherein the applying means further comprises means for applying a gas inthe vicinity of the at least one impurity particle, wherein the gas andparticle are at temperatures such that the liquid condenses from the gasin the vicinity of the at least one impurity particle.
 15. The apparatusof claim 13, wherein the means for explosively evaporating the liquid isa laser apparatus.
 16. The apparatus of claim 13, further comprisingmeans for applying a suction to the vicinity of the at least oneimpurity particle to assist in particle removal upon evaporation. 17.The apparatus of claim 13, further comprising means for monitoring theexplosive evaporation for particle removal.
 18. The apparatus of claim13, wherein the liquid applying means includes a source of water or awater-containing composition, or alcohol, or an alcohol-containingcomposition.
 19. The apparatus of claim 13, wherein the material is asilicon wafer, and means for handling the silicon wafer are provided tointerface with the liquid applying means.
 20. The method of claim 11,wherein steps (b) and (c) are repeated one or more times if themonitoring step indicates that the particle has not been removed.